The herbicides most commonly used in Turkey are trifluralin (alpha,alpha,alpha-trifluoro-2,6-dinitro-NN-dipropyl-p-toluidine), 2,4-D(2,4dichlorophenoxyacetic acid), and linuron [3-(3,4-dichlorophenyl)-N-methoxy-N-methylurea]. The effects of these three herbicides on the growth of 10 threatened aquatic cyanobacterial isolates were tested by using 9-day exposure experiments at concentrations of 50-200 mg l(-1) for trifluralin and 2,4-D and 0.05-1 mg l(-1) for linuron. Concentrations of herbicides that elicited a 50% growth reduction over 9 days (EC50) were 136-882 mg l(-1) trifluralin, 122-747 mg l(-1) 2,4-D, and 0.002-0.714 mg l(-1) linuron. Synechocystis sp. H-6 was more tolerant to the three herbicides than the other isolates of cyanobacteria. Chroococcus sp. S-27, Microcystis sp, S-17, and Synechococcus sp. S-24 were the most sensitive to trifluralin, 2,4-D, and linuron, respectively. There has been increasing awareness about using cyanobacteria as pollution control agents. The present study indicated that as the concentrations of the these herbicides were increased, significant changes were recorded in cyanobacterial growth rates. Results obtained from this comparative study allow the choice of suitable herbicides for agricultural practices based on their effects on cyanobacterial growth.